This invention relates to railroad cars, and more particularly to protecting covered hopper cars from vacuum failure during unloading.
A covered hopper cars typically has partitions dividing the car into separate compartments, with each compartment having one or more openings for loading the car. The openings are typically defined by a coaming which extends upward on the top wall of the car, and may comprise, e.g., elongated rectangular openings or circular hatches. Where vacuum discharge gates are employed, materials are typically unloaded by applying a vacuum conveying line to a nozzle at the bottom of each compartment. The primary air flow for the vacuum line comes from outside the car, but the displacement of the lading creates a vacuum within the car, requiring venting of the car. If the partial vacuum is not relieved, it can reach a point where the car's body is damaged. Failure to vent the vacuum also reduces the efficiency of the unloading process. Where gravity or gravity pneumatic gates are employed, similar problems exist.
In the past, venting during unloading to prevent vacuum failure has commonly been achieved by opening at least one hatch cover on each compartment. However, this is somewhat inefficient in that it requires workers to climb to the top of the car to manually open each hatch. Venting may also be required after cleaning. If a compartment is cleaned with hot water or steam and the hatch is closed and sealed before the car interior has cooled to ambient temperature, subsequent cooling may result in a temperature differential sufficient to cause inward buckling of the walls of the car.
In recent years, two alternatives have been explored for venting of covered hopper cars: vents on the end walls of the cars and vented hatch covers. Each approach has problems.
Among the problems that must be addressed in providing a commercially viable vent in a hatch cover or end wall are removal of contaminants from the airflow during unloading, and physical access for maintenance. If filters or screens are used over the vents, sufficient physical access must be provided to enable railroad personnel to change or maintain them without undue difficulty. Consideration must also be given to the railcar's exposure to high winds, and high rates of airflow relative to the hatch covers during travel. Precipitation, dust, and other particles may be driven into the cars, contaminating the cargo, absent adequate filtration, particularly during conditions of high winds, rain, snow or other inclement weather.
Another problem that must be addressed by any venting solution for cars where high standards of purity apply, such as cars carrying plastic pellets, is preventing retention of any lading material. Retention of even a small quantity of a load in a vent, hatch cover or other component of a railcar may result in contamination of a subsequent load of a different material.
Vented hatch covers have not generally included filters capable of preventing fine particulates from contaminating the cargo. Accordingly, separate filters have generally been used to cover the openings underneath the vented hatch covers. These filters are similar in configuration to shower caps, with elastic being provided to hold them in place to cover the hatches after the covers have been raised. A significant disadvantage of these filters is that they must be removed when loading material into the car interior, and may have to be replaced with each load. It has been difficult to provide a commercially viable alternative due at least in part to the difficulties in providing a filter arrangement which can provide required volume flow rate during unloading without violating overhead clearance requirements, and which can be easily changed or serviced. Limited space is available adjacent the hatches, so bulky filter arrangements cannot be used. However, if the cross-section of the airflow path through a filter element is not large enough, the pressure drop across the filter element may be unacceptably high and cause inward buckling of the railcar sidewalls in response to vacuum build-up in the railcar interior.
Additional problems with vents in the end walls of covered hopper cars are that they provide airflow only to the end compartments, and due to difficult access, are very likely not to be maintained, thereby endangering the car body when the filter becomes clogged. Some structure may be required to support workers conducting maintenance. In the past, small breather holes of about 1 in. diameter have been provided on the ends of covered hopper cars carrying cement. These cars do not have seals between their interior compartments. A piece of angle or other protective structure is welded to the wall adjacent the hole to shield it from rainwater. Small breather holes have also been provided in hatch coamings, in conjunction with wire mesh to exclude insects. The breather holes permit sufficient airflow to compensate for changes in ambient temperature, but do not permit sufficient airflow to replace displaced cargo volume. Also, many cars have sealed interior compartments which cannot be vented by end wall vents.
There is a need in the industry for an improved commercially viable means to enable unloading of covered hopper cars which avoids the need to open the hatches during unloading, and which avoids contamination of the cargo both during vacuum discharge and at other times.